The present invention generally relates to an interferometric optical system and, in particular, relates to a conjugate interferometer the common optical element of which is a moving mirror.
In general, an interferometer is any arrangement whereby a beam of light is separated into two or more parts by partial reflections, the parts being subsequently reunited after traversing different optical paths. The reunited parts produce interference. Interferometers are used, inter alia, for the precise measurement of wavelengths and for the measurements of very small distances and thicknesses by using known wavelengths.
In recent years interferometers have been increasingly used in the field of infrared spectrophotometry and, in particular, in instruments generally referred to as Fourier Transform Infrared (FT/IR) spectrophotometers. By the use of an interferometer, rather than the more conventional dispersive optical system, an infrared analysis can be performed faster, with greater light energy throughput and with a reduction in errors caused by stray light. These advantages are well known in the analytical instrument field.
However, in conventional FT/IR instruments there is one disadvantage, particularly in comparison to dispersive instruments. The disadvantage is that an interferometer has a reduced, i.e. narrower, range of operating wavelengths. In dispersive instruments the range of operating wavelengths is comparatively broad primarily because of the use therein of easily interchanged diffraction gratings which disperse incoming light according to wavelength. These diffraction gratings are generally made to rotate and thus direct a particular narrower, or dispersed, band of wavelengths of interest towards the sample. The range of wavelengths over which an interferometer functions is determined by the beamsplitter thereof. As it happens, the presently available beamsplitters, although as a group provide a broad range of operating wavelengths, individually provide a comparatively narrower range of operating wavelengths.
One solution to the difficulties caused by this transmission characteristic of beamsplitters is to fabricate an optical interferometer wherein the beamsplitter is removable. In such an interferometer various beamsplitters could be interchanged to provide a relatively broad range of operating wavelengths. However, to obtain highly accurate results the mechanical tolerances on the beamsplitters and the associated fixtures would be difficult and expensive to maintain. This would become more difficult in the long term due to the natural wearing of interfacing mechanical parts. This inherently results in optical alignment difficulties.
A further difficulty results from the fact that the optical compartment of an FT/IR spectrophotometer is usually maintained in a controlled environment. Consequently, each time a beamsplitter is exchanged the compartment is subject to contamination as well as necessitating the reestablishment of the controlled environment. This is inefficient and time consuming.